ASX-listed Chilwa Minerals has highlighted the prevalence of widespread rare earth element mineralisation in clay intervals directly beneath the Mposa heavy minerals sands (HMS) deposit, in southern Malawi.

Assays from 200 mineral sands drilling samples have revealed total rare earth oxide (TREO) grades averaging 525 parts per million (ppm).

Chilwa says the samples have delivered consistent and widespread grades, with 114 of the 200 samples having returned a TREO greater than 500 ppm, with 17 samples exceeding 750 ppm TREO.

Notably, the assay revealed a peak grade of 987 ppm TREO, pointing to broadly consistent mineralisation across an 8 km corridor.

The mineralised clays were sampled during regular HMS drilling and samples were collected from the top of the clay unit, which is known to extend to great depths.

“These are exciting results for Chilwa. We set out to test whether the clays beneath our Mposa mineral sands deposit carried rare earths, and the assays have come back confirming widespread rare earth elements mineralisation.

“The basket is encouraging too, with the valuable magnet rare earths – neodymium, praseodymium, dysprosium and terbium – making up [about] a fifth of the total. The real prize would be if these rare earths prove to be held in an ionic-adsorption clay form, the style behind some of the world's lowest-cost rare earth operations,” says Chilwa MD Cadell Buss.

He concedes that these results are from early-stage reconnaissance work and do not yet qualify as a mineral resource. However, another promising exploration target has been identified by the company and it plans to begin desorption testwork to determine whether the Mposa deposit, together with the larger Mpyupyu and other HMS deposits on the licence, also contain clay-hosted rare earth mineralisation beneath the HMS.

Additionally, the assay results also highlight that heavy rare earth oxides average about 19.6% of the TREO and that high-value magnet rare earth oxides, which include neodymium, praseodymium, dysprosium and terbium, average about 21% of the TREO, equating to an average of about 102 ppm neodymium-praseodymium.

Chilwa states that a heavy magnet rare earth proportion of this order is characteristic of clay-hosted systems and is significant as these elements carry the majority of rare earth basket value.

At present, no metallurgical work has been undertaken and, therefore, the proportion of the contained rare earths present in a recoverable, ionic-absorbed form is unknown.

Initial metallurgical testwork, which includes ammonium-sulphate leach testing of the clay sample composites, will be undertaken by research institute the Australian Nuclear Science and Technology Organisation (ANSTO) in Sydney, Australia.

This will determine the proportion of the contained rare earths recoverable in ionic-absorbed form, as it is viewed as the key value driver for a clay-hosted system.

Further confirmatory Inductively Coupled Plasma Mass Spectrometry, or ICP-MS, before and after leach testwork at ANSTO will also be included in the test scope.

Drilling will also be planned to depth at the other HMS deposits on the licence, especially at Mpyupyu, supported by clays and encompassing an area of 15 km².